1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
Conventionally, an image scanner is used as an image reading means in duplicators, facsimiles, OCRs, filing systems, etc. The image scanner includes a light source for emitting light along an incident optical axis toward an original reading position in an original transport path, a detector such as a CCD arranged in such a manner as to detect the light irradiated from the light source and reflected along a reflection optical axis at the original reading position, and an image processing means connected to the detector. Also, a white backing member is arranged in the original transport path.
In recent years, the trend has been toward a thinner original or paper used as a medium. Also, the documents used in offices have more frequently come to be printed on both surfaces thereof. In reading the image on one of the surfaces of a thin original printed on both surfaces thereof, the image printed on the other surface (reverse side) of the original may be seen through the original and detected by the reading apparatus so that the image on the reverse side may be output overlapped on the image on the front surface, resulting in a deteriorated image quality.
The reading of the image on the reverse side of the original is caused by the fact that the light passes through the original, is reflected by the white backing member, returns through the original and is detected by the detector. Consequently, whenever the image on the reverse side of the original is detected from the front surface of the original, the image on the reverse side is comparatively thin.
In view of this, the image on the reverse side can be prevented from being output by correcting the slice level or the Gamma curve for binarizing the image when the output signal of the detector is image-processed and by judging the detection value of the level of a low light amount, corresponding to the image on the reverse side of the original, as white. In the case where the entire image on the original is comparatively thin, however, the thin image on the front surface of the original may also be judged as white, thereby leading to the problem that the required image drops off for a reduced image quality and the image on the front surface which should otherwise be read cannot be read. Especially, the output image of a thin line or a thin character may be greatly affected.
Another possible means for preventing the image on the reverse side of the original from being output is to make the surface of the backing member black. Making the surface of the backing member black reduces the reflectance of the backing member and therefore the amount of the light passing through the original and reflected by the backing member is reduced, so that the image on the reverse side of the original cannot be seen through the original and is not detected by the detector. In the case where the width of the original is smaller than that of the backing member, however, the backing member extends out of the original, thereby posing the problem that a portion of the backing member is read as a black image.
Japanese Unexamined Patent publication No. 60-74765 discloses an image input apparatus in which the reflectance of the backing member is variable. According to this prior art, the backing member is formed of a liquid crystal device or a movable belt having two portions of different reflectances. The apparatus judges whether the reverse side of the original is white or printed. In the case where the reverse side of the original is white, the reflectance of the backing member is increased, while in the case where the reverse side of the original is not white, the reflectance of the backing member is reduced. This backing member, however, is complicated in structure.
Japanese Unexamined Patent publication No. 63-102452 discloses a reading apparatus in which a guide corresponding to the backing member is formed with a slope entered by the light from a light source and in which the light reflected from the slope mostly enters a detector. The light amount entering the detector is increased in order to prevent the backing member portion around the original from being detected as black when the amount of the light incident to the detector is small. This prior art is not intended to prevent the image on the reverse side of the original from being read when reading the image on one of the two printed surfaces of a thin original.